Low Recirculation Rate Research Articles

Innovative organic waste pretreatment approach for efficient anaerobic bioconversion: Effect of recirculation ratio at pre-processing in vortex layer apparatus on biogas production

Agricultural waste, called biomass, is of great interest as a renewable energy source. The purpose of this work was to determine the optimal recirculation rate (RR) values of the solid digestate when applying a new approach to the intensification of anaerobic conversion, providing a high quality biogas yield. This approach includes solid digestate recirculation and pre-processing a mixture of raw waste and solid digestate in a vortex layer apparatus. For this, the operation of an experimental biogas plant without recirculation and with recirculation was considered, starting from very low RR values, from 0.034 to 1, to ensure a high methane production rate (MPR) and methane yield (MY). From the point of view of MPR and MY, the recirculation ratios of 0.034 and 0.071 look the most attractive, however, with a recirculation ratio of 0.5, the content of hydrogen sulfide de-creased to 14 ppm with a simultaneous increase in MPR by 12 % compared to the control.

MO787: Effects of the End Structure of Temporary Indwelling Catheters on Blood Recirculation under Varying Blood Flow Rates of a Dialysis Machine and Diameters of a Mock Blood Vessel

Abstract BACKGROUND AND AIMS The performance of temporary indwelling catheters allowing higher recirculation during hemodialysis may result in reduced clearance and inadequate dialysis and consequently, reduced patient survival. In actual clinical practice, it is often necessary to use a reverse connection to recover the blood flow rate for dialysis when there is inadequate blood removal during the treatment. The aim of the present study was to determine the effects of the end-structure of catheters used for hemodialysis on blood recirculation under varying blood flow rates of a dialysis machine and diameters of a mock blood vessel in a well-defined in vitro experimental system, focusing on the reverse connection. METHOD Pig blood was circulated by a roller pump through a mock vessel made of silicon tubes with an inner diameter of 15 mm or 20 mm. The tip of the inserted catheter was fixed at the center of a silicone tube. We connected the catheter to a dialysis machine, a blood circuit and a dialyzer. The blood flow rate of the dialysis machine was set at 100, 150, or 200 mL/min and the recirculation rates were determined. The recirculation rate was measured three times under each condition, and the average value was determined. Five types of catheters were used; Argyle TM 12 Fr × 25 cm (catheter A), Gentle Cath TM (Hardness gradient type) 12 Fr × 13 cm (catheter B), Gentle Cath TM 12 Fr × 24 cm (catheter C), Niagara TM 13.5 Fr × 24 cm (catheter D), Power-Trialysis® 13 Fr × 24 cm (catheter E). Catheters A, B, C and D have asymmetric structures with a side hole, while E has a symmetric structure. RESULTS The recirculation in the sequential connection was very low for all catheters. The recirculation rates in the reverse connection for catheters A, B, C, D and E were 40%, 25%, 17%, 25% and 10%, respectively. The recirculation rates were the lowest when the blood flow rate of the dialysis machine was set at 200 mL/min and the inner diameter of the mock blood vessel was 20 mm. DISCUSSION Recirculation was most likely to occur for catheter A. The distance from the end hole to the side hole was longer in catheters B, C and D than in catheter A, which could explain the lower recirculation rates. Among the catheters, catheter C had the largest side hole, and the blood spread widely into the vessel, leading to a lower recirculation rate. Therefore, the distance between the end hole and the side hole and the size of the end hole was deduced as being essential factors for reducing the recirculation in the case of reverse connection. For catheter E, the blood flow line in the mock vessel coming from the dialysis machine through the blood returning hole (side hole) was helical, and little blood existed near the blood withdrawal hole (the other side hole), which could be responsible for the reduced recirculation rate. CONCLUSION The least recirculation in the reverse connection was observed with symmetrical catheters, where the blood streamlines were spiral and did not intersect with the blood streamline between the blood withdrawal hole and the blood returning hole.

Innovative reactive distillation process for the eco-friendly Poly(oxymethylene) dimethyl ethers synthesis from methylal and trioxane

• Improved performance of PODEn selectivity by reactive distillation technology. • A rigorous simulation model of the RD process was developed for the plant design. • The RD process for PODEn synthesis is assessed from techno-economic views. • Proposed the regulation mechanism of the parameters on the polymerization degree. Poly(oxymethylene) dimethyl ethers (PODE n ) is a promising excellent additive for diesel fuels. The PODE n production by traditional reactor conform to Schulz − Flory distribution resulting in low product yield and large circulating material flow, which leads to high energy consumption and cost. This study presents a novel reactive distillation (RD) process for the PODE n synthesis by polymerization reaction from trioxane and methylal. A rigorous simulation model of the RD process was developed for the plant design and scale-up. Also, a sensitivity analysis was performed to determine the influence of key operating parameters on the RD process performance. The production composition analysis indicates that the RD process can effectively control the degree of polymerization and obtain high selectivity for any target product(s). The techno-economic analysis shows that the process is feasible, with over 96% and 13% reduction in CapEx and OpEx, over 68% savings in total annual costs as compared to the reaction-separation technology. Higher conversion and selectivity can be obtained by RD, within lower recirculation rate (only 12281 kg/h instead of 2950 kg/h) as compared to the conventional process.

Effect of Operating Conditions on Membrane Fouling in Pilot-Scale MBRs; Filaments Growth, Diminishing Dissolved Oxygen and Recirculation Rate of the Activated Sludge.

This is the first study that examines the effect of operating conditions on fouling of Membrane Bio-Reactors (MBRs), which treat municipal wastewater in field conditions, with specific regard to the controlled development of filamentous microorganisms (or filaments). The novelty of the present work is extended to minimize the dissolved oxygen (DO) in recirculated activated sludge for improving the process of denitrification. For this purpose, two pilot-scale MBRs were constructed and operated in parallel: (i) Filament-MBR, where an attempt was made to regulate the growth of filaments by adjustment of DO, the Food-to-Microorganisms (F/M) ratio and temperature, and (ii) Control-MBR, where a gentle stirring tank was employed for the purpose of zeroing the DO in the recycled sludge. Results showed that low temperature (<15 °C) slightly increased the number of filaments in the Filament-MBR which, in turn, decreased the Trans-Membrane Pressure (TMP). As the Soluble Microbial Products (SMP) and the colloids are considered to be the basic foulants of membranes in MBR systems, specific attention was directed to keep their concentration at low values in the mixed liquor. The low F/M ratio in the aeration tanks which preceded the membrane tank was achieved to keep the SMP proteins and carbohydrates at very low values in the mixed liquor, i.e., less than 6 mg/L. Moreover, as a result of the low recirculation rate (2.6∙Qin), good aggregation of the produced excess sludge was achieved, and low concentration of colloids with a size ≤50 nm (nearly the membranes’ pore size used for filtration/separation) was measured, accounted for maximum 15% of the total colloids. Additionally, the increase in filamentous population at the Filament-MBR contributed to the further reduction of colloids in the mixed liquor at 7.9%, contributing beneficially to the reduction of TMP and of membrane fouling. The diminishing of DO in the recirculated sludge improved denitrification, and resulted in lower concentrations of Ν-NO3− and TN in the effluent of the Control-MBR. Furthermore, the recirculation rate of Qr = 2.6∙Qin, in comparison with Qr = 4.3∙Qin, resulted in improved performance regarding the removal of N-NH4+. Finally, high organics removal and ammonium nitrification was observed in the effluent of both pilots, since COD and Ν-ΝH4+ concentrations were generally in the range of 10–25 mg/L and <0.1 mg/L, respectively.

Development of New Hemodialysis Catheter Using Numerical Analysis and Experiments.

A hemodialysis (HD) catheter, especially one with a symmetric tip design, plays an important role in the long-term treatment of patients with renal failure. It is well known that the design of the HD catheter has a considerable effect on blood recirculation and thrombus formation around it, which may cause inefficiencies or malfunctions during HD. However, hemodynamic analyses through parametric studies of its designs have been rarely performed; moreover, only comparisons between the existing models have been reported. In this study, we numerically analyzed the design of the HD catheter's side hole and distal tip for evaluating their effects on hemodynamic factors such as recirculation rate (RR), shear stress, and blood damage index (BDI). The results indicated that a larger side hole and a nozzle-shaped distal tip can significantly reduce the RR and shear stress around the HD catheter. Furthermore, based on these hemodynamic insights, we proposed three new HD catheter designs and compared their performances with existing catheters using numerical and in vitro methods. These new designs exhibited lower RRs and BDI values, thus providing better performance than the existing models. These results can help toward commercialization of more efficient HD catheters.

Denitrification Control in a Recirculating Aquaculture System—A Simulation Study

The recirculating aquaculture system (RAS) is a land-based water treatment technology, which allows for farming aquatic organisms, such as fish, by reusing the water in the production (often less than 5%). This technology is based on the use of filters, either mechanical or biological, and can, in principle, be used for any species grown in aquaculture. Due to the low recirculation rate, ammonia accumulates in the system and must be converted into nitrate using nitrification reactors. Although less toxic for fish, nitrate can also be further reduced into nitrogen gas by the use of denitrification biofilters which may create several issues, such as incomplete denitrification, resulting in toxic substances, such as nitrite and nitric oxide, or a waste of carbon source in excess. Control of the added quantity of carbon source in the denitrification biofilter is then mandatory to keep nitrate/nitrite concentrations under toxic levels for fish and in accordance with local effluent regulations, and to reduce costs related to wasted organic carbon sources. This study therefore investigates the application of different control methodologies to a denitrification reactor in a RAS. To this end, a numerical simulator is built to predict the RAS behavior and to allow for the comparison of different control approaches, in the presence of changes in the operating conditions, such as fish density and biofilter removal efficiency. First, a classical proportional-integral-derivative (PID) controller was designed, based on an SIMC tuning method depending on the amount of ammonia excreted by fish. Then, linearizing and cascade controllers were considered as possible alternatives.

Formation and use of biogenic jarosite carrier for high-rate iron oxidising biofilms

Jarosite precipitates formed in iron oxidising bioreactors have been shown to harbour iron-oxidisers. The aim of this study was to develop an iron oxidising bioprocess where microorganisms are retained solely on biogenic jarosite particles. Based on preliminary experiments using a fluidised-bed bioreactor (FBR), the formed jarosite particles started to disintegrate and wash out at upflow velocities of ≥0.21 cm/s. Therefore, the generation and use of biogenic jarosite carrier was studied in an expanded-bed bioreactor (J-EBR) with an upflow velocity of 0.19 cm/s. Inside J-EBR, the jarosite particles formed granules of 0.5–3 mm containing 200–460 mg/g of attached biomass. The performance of J-EBR was compared with an activated carbon biofilm FBR at 0.82 cm/s upflow velocity (AC-FBR). At 35 ± 2 °C with a feed ferrous iron concentration of 10 g/l, the highest obtained iron oxidation rate of J-EBR (6.8 g/l/h) was 33% lower than that of AC-FBR (10.1 g/l/h). This was likely due to the 80% lower recirculation rate and subsequently higher oxygen mass transfer limitation in J-EBR compared to AC-FBR. The present study demonstrates that biogenic jarosite can be used for retainment of iron oxidising biofilms in expanded-bed bioreactors that oxidise iron at high rates.

Assessment of air gap membrane distillation for milk concentration

Multi-effect evaporation is the state of the art for concentration of liquid food products to high solid content. Membrane technology with reverse-osmosis and membrane distillation offer an alternative. For the concentration of milk, a reverse osmosis and air-gap membrane distillation network was modelled and optimized. Fouling dynamics and scheduling are taken into account. Reverse osmosis is favourable until its maximum achievable concentration. Air gap membrane distillation is, despite the low operational temperatures, energy intensive for the concentration of milk. A large recirculation flow to keep sufficient cross flow has to be heated and cooled, and the costs for heating and cooling dominate the total costs for product concentration. Moreover, fouling increases the energy requirements. The optimal system for air gap membrane distillation has only one stage operating at a high concentration and relative low flux. Applying multiple stages reduces the investment costs due to smaller units, but the heating and cooling costs increase. Major opportunities to improve the performance of air gap membrane distillation for concentration of milk are: 1) increase the cold and hot side temperatures to their maximum acceptable values, 2) develop spacers that allow lower linear flow velocities in the system and thus lower recirculation rates, and 3) make use of available waste heat.

Dissolved air flotation (DAF) for biomass recovery from algal ponds

This work evaluated the performance of a dissolved air flotation (DAF) system for biomass recovery from the effluent of algal ponds used for sewage treatment in Northeast Brazil. Two different coagulants and effluent samples were tested following a random-factor experimental design (34–1–4 factors and 3 levels), in triplicate. Factor analysis showed that coagulant dose was the most important factor, independently of the type of coagulant and sample characteristics. For polyaluminium chloride it is possible to work with the original sample pH at the lower recirculation rate (10%) and flocculation time (10 minutes), being able to recovery an algal masses (dry basis) of around 1,200 kg/day and 550 kg/day from the primary facultative and second maturation pond (final effluent), respectively. DAF was more efficient for ferric chloride, however a pH correction to the value of 5.0 was necessary, leading the process to generate masses of solids (dry basis) of about 1,150 kg/day for the facultative pond and 830 kg/day for the maturation pond. Removals of suspended solids, turbidity, and chlorophyll-a were above 60%, reaching at optimal conditions removals up to 90%.

Recirculation and Aeration Effects on Deammonification Activity

Deammonification process has been studied as an alternative technology for nitrogen removal. This process consists of the association between nitrifying and anammox bacteria, in which the process success is related to aeration, recirculation, and reactor configuration. Considering this, the present study aimed to evaluate the performance of an expanded granular sludge bed (EGSB) reactor on nitrogen removal by deammonification process. Established in a single reactor, it considered the effects of recirculation rate and variation of dissolved oxygen (DO) concentration in microbial community and nitrogen removal efficiency. Thus, two independent tests were conducted: (T1) high recirculation flow rate, performed at 43 L d−1 (Qr/Qin = 16), aeration of 30 mLair min−1 L−1reactor, and conducted during 16 days; (T2) low recirculation flow rate performed at 6.7 L d−1 (Qr/Qin = 2.5), operated for 55 days, divided into three aeration phases: (T2a) 30 mLair min−1 L−1reactor, (T2b) 20 mLair min−1 L−1reactor, and (T2c) 30 mLair min−1 L−1reactor. Results showed that in T1 the high recirculation rate favored nitrifying bacteria prevalence, intensified by reactor turbulence and anammox granules disintegration, changing activity from deammonification to a nitrification process. In addition, T1 reached up to 350 ± 100 mgN L−1 d−1 nitrogen removal rate (NRR). For T2, at low recirculation rate, deammonification process was successfully established with a NRR of 490 mgN L−1 d−1 at Qr/Qin = 2.5 and air flow rate of 20 mLair min−1 L−1reactor.

Analysis of a Solid Oxide Fuel Cell System with Low Temperature Anode Off-Gas Recirculation

At Forschungszentrum Jülich an SOFC system with anode off-gas recirculation loop operating at temperatures up to 160 °C is simulated. To analyze the effect of anode off-gas recirculation, a dynamical system model was implemented in Matlab/Simulink®. The results show, that the recirculation rate has a significant effect on the electrical efficiency. In principle, at constant current density high recirculation rates decrease the cell voltage and increase the power demand of the recirculation blower. Therefore, the highest electrical efficiency can be reached with high system fuel utilization, low recirculation rate and in consequence high stack fuel utilization. On the other hand less amount of steam is available for the reforming reaction at low recirculation rates. Therefore, the minimum recirculation rate is determined by the risk of carbon formation. An optimal operation range to avoid carbon formation and to ensure a high electrical efficiency requires recirculation rates between 65 and 70 % and system fuel utilizations above 90 %.

Analysis and optimization of a coal-fired power plant under a proposed flue gas recirculation mode

This study mainly investigated the influence of flue gas recirculation (FGR) on reheat steam temperature, boiler efficiency, and thermal efficiency in a proposed FGR scheme. The main parameters of the boiler are calculated under different modes on the basis of a 600MW coal-fired boiler. Optimization is conducted, and the optimum recirculation rate and coal input are determined. Results from case study showed that under the same recirculation rate, the reheat temperature increases higher and the net coal consumption decreases more in the improved recirculation system than those in the traditional recirculation system. The key parameters, such as the FGR rate and coal feed rate are optimized. Under optimum recirculation rate and coal input conditions, the net coal consumption rate can be reduced by 2.35–2.60g/(kWh) compared with that obtained with the use of a conventional recirculation system. This net coal consumption rate can be reduced by 3.50–5.11g/(kWh) compared with that obtained without gas recirculation system. For a better operation, lower recirculation rate and relative higher coal input are recommended under high-load conditions, whereas higher recirculation rate and relative lower coal input are appropriate for low-load conditions.

Analysis of a Solid Oxide Fuel Cell System with Low Temperature Anode Off-Gas Recirculation

At Forschungszentrum Jülich GmbH an SOFC system with anode off-gas recirculation loop operating at temperatures up to 160°C is simulated. To analyze the effect of anode off-gas recirculation, a dynamical system model was implemented in Matlab/Simulink. The results show, that the recirculation rate has a significant effect on the electrical efficiency. In principle, at constant current density high recirculation rates decrease the cell voltage and increase the power demand of the recirculation blower. Therefore, the highest electrical efficiency can be reached with high system fuel utilization, low recirculation rate and in consequence high stack fuel utilization. On the other hand less amount of steam is available for the reforming reaction at low recirculation rates. Therefore, the minimum recirculation rate is determined by the risk of carbon formation. An optimal operation range to avoid carbon formation and to ensure a high electrical efficiency requires recirculation rates between 65 and 70% and system fuel utilizations above 90%.

바이오가스에 포함된 고농도 황화수소의 효율적 제거를 위한 미생물반응기

A two-stage bioreactor system using sulfur-oxidizing bacteria was studied to abate high strength hydrogen sulfide () from biogas. The two-stage bioreactor consisted of a absorption column (0.5 L) and a microbial oxidation column (1 L) in series, and the liquid medium was continuously recirculated through the columns. The objectives of this study were to determine the feasibility of the bioreactor for biogas desulfurization and to investigate the effect of the medium circulation rate on the system performance. An averaged concentration of introduced to the bioreactor was 530 ppm, corresponding to an overall loading rate of . During the initial 20 days period at the medium recirculation rate of 8 reactor volumes per hour (12 L/hr), the dissolved oxygen (DO) concentration in the oxidation column was 6 mg/L, while the DO in the absorption column was 0.5 mg/L showing that the oxygen contents of the biogas stream was not altered. Because of the biological oxidation of in the oxidation column, the sulfate concentration increased from 200 mg/L to 5,600 mg/L in the liquid medium. The removal efficiency of was greater than 99% in the initial operation period. After the initial period, the medium recirculation rate between the two columns was stepwise changed eight times from 1.0 to 40 vol/hr (1.5~60 L/hr). At the recirculation rate of faster than 4 vol/hr, the removal efficiencies were found to be high, but the efficiency declined at the lower recirculation rates than the threshold.

Improving hydrolysis of food waste in a leach bed reactor

This paper examines the rate of degradation of food waste in a leach bed reactor (LBR) under four different operating conditions. The effects of leachate recirculation at a low and high flow rate are examined with and without connection to an upflow anaerobic sludge blanket (UASB). Two dilution rates of the effective volume of the leach bed reactors were investigated: 1 and 6 dilutions per LBR per day. The increase in dilution rate from 1 to 6 improved the destruction of volatile solids without connection to the UASB. However connection to the UASB greatly improved the destruction of volatile solids (by almost 60%) at the low recirculation rate of 1 dilution per day. The increase in volatile solids destruction with connection to the UASB was attributed to an increase in leachate pH and buffering capacity provided by recirculated effluent from the UASB to the leach beds. The destruction of volatile solids for both the low and high dilution rates was similar with connection to the UASB, giving 82% and 88% volatile solids destruction respectively. This suggests that the most efficient leaching condition is 1 dilution per day with connection to the UASB.

Oxyfuel combustion of lignite in a non-stoichiometric operating two burner arrangement

Next to the recirculation rate and that way the oxygen concentration in the oxidizer, the stoichiometry offers an option to control flame temperatures in oxyfuel processes. While using both, non-stoichiometric flames and recirculated flue gas as in the concept of controlled staging with non-stoichiometric burners (CSNBs), the recirculation rate could be reduced drastically. Reducing the recirculation rate in the oxyfuel process is a viable option for more efficient steam generator concepts. The auxiliary power demand, the size of the recirculation equipment and the steam generator size is reduced. The investigations in this paper focus on the interaction of two non-stoichiometric operating pulverized fuel oxycoal burners as used in the CSNB concept. Two different staging strategies have been investigated in detail as well as the influence of dry and wet flue gas recirculation. For that purpose species concentrations have been measured along the flue gas path in the combustion chamber. Additionally emissions at the combustion chamber exit as well as mass and energy balances of the process are reported. The species concentration profiles demonstrate the functionality of the CSNB concept. Either way the oxygen excess or the hydrocarbon excess of the first flame level could be balanced by the contrarily operating second flame level. The staged CSNB concept with low recirculation rate shows comparable combustion performance with the unstaged oxyfuel process with high flue gas recirculation. However the influences of the individual burner performance and the flue gas mixing, especially in the second burner level, is emphasized in the CSNB concept. In concerns of NOx emissions shows the arrangement, first sub- then over-stoichiometric combustion lower values because the initial NO formation in the first level is much lower. In case of over- and then sub-stoichiometric arrangement the NO reduction by the second sub-stoichiometric flame is low. In case of wet flue gas recirculation with increased vapor concentration in the flue gas atmosphere the heat transfer in the combustion chamber is slightly higher compared to the dry oxyfuel case.

A combustion concept for oxyfuel processes with low recirculation rate – Experimental validation

Oxyfuel combustion is a technology for Carbon Capture & Storage from coal fired power plants. One drawback is the large necessary amount of recirculation of cold flue gases into the combustion chamber to avoid inadmissible high flame temperatures. The new concept of Controlled Staging with Non-stoichiometric Burners (CSNB) makes a reduction of the recirculation rate possible without inadmissible high flame temperatures. This reduction promises more compact boiler designs. We present in this paper experiments with the new combustion concept in a 3 × 70 kW natural gas combustion test rig with dry flue gas recirculation of 50% of the cold flue gases. The new concept was compared to a reference air combustion case and a reference oxyfuel combustion case with recirculation of 70% of the cold flue gases. FTIR emission spectroscopy measurements allowed the estimation of spectral radiative heat fluxes in the 2–5.5 μm range. The mixing of the gases in the furnace was good as the burnout and the emissions were comparable to the reference cases. The flame temperatures of the CSNB case could be controlled by the burner operation stoichiometry and were also similar to the reference cases. The heat flux in the furnace through radiation to the wall was higher compared to the oxyfuel reference case. This is an effect of the lowered recirculation rate as the mass flow out of the furnace and therefore the sensible heat leaving the furnace decreases. The higher oxygen consumption with lower recirculation rate could be compensated by a lower furnace stoichiometry. This was possible due to better burnout with increased oxygen concentrations in the burner. The results prove that a reduction of the flue gas recirculation rate in oxyfuel natural gas combustion from 70% down to 50% is possible while avoiding inadmissible high flame temperatures with the concept of Controlled Staging with Non-stoichiometric Burners.

Effect of hydrodynamic conditions on fractionation of β-lactoglobulin tryptic peptides using nanofiltration membranes

Fractionation of a β-lactoglobulin (β-LG) peptide mixture by nanofiltration (NF) membranes was investigated. Peptide mixture was prepared by tryptic hydrolysis of commercial β-LG followed by ultrafiltration (UF) for enzyme removal. The peptide mixture (permeate) was further fractionated using an Osmonics-SG13 cellulose acetate NF membrane with a molecular weight cut-off (MWCO) of 2500 g mol −1. The effect of hydrodynamic parameters on a “resistance-in-series” model and transmission of tracer peptides was studied at 0.5 wt.% hydrolysate concentration by varying the operating pressure (0.200 and 0.500 MPa), recirculation rate (0.17 and 0.29 m s −1), and recirculation time (30 and 120 min). A detailed comparison on individual peptide transmission revealed that increasing recirculation time to 120 min, at lower recirculation rates, greatly affected peptides β-LG 102–105 and β-LG 142–148 at both operating pressures. Our results are indicative of important changes in NF selectivity as affected by hydrodynamic conditions and recirculation time via possible peptide–peptide interactions occurring in the so-called weakly attached layer.